It is the purpose of this project to study the mode of action and the pharmacology of antitumor agents of the ansa macrolide class. Attention is presently being concentrated on the plant product maytansine and its naturally occurring and semi-synthetic derivatives. Maytansine was found to impair microtubule formation in mammalian cells; and to produce metaphase arrest similar to that seen with the Vinca alkaloids. Topics of present interest are a) the nature of the maytansine binding site; b) electron microscopic studies of the alterations in microtubular structure produced by maytansine; c) bioassay methods for maytansine.